1. Field of the Invention
The present invention relates to a process control device and method of adjustment thereof that control subjects of control in for example a power-generating plant or typical industrial plants such as petrochemical or iron and steel plants so that a prescribed control performance is obtained and in particular relates to a process control device, method of adjustment thereof and adjustment tool therefor whereby adjustment of the control parameters of control items may be performed in a simple fashion.
2. Description of the Related Art
Conventionally, in order to control the process values of subjects of control such as temperature, flow rate, pressure or liquid level to prescribed values in for example power-generating plants or typical industrial plants (general industrial plants) such as petrochemical or iron and steel plants, a PID control device combining proportional, integral and differential operations was widely employed.
In a conventional PID control device, if the characteristics of the subjects of control are lagging characteristics, excellent control performance may be obtained and many adjustment rules for the control parameters of such PID control devices have been proposed.
However, there is the problem that, if the subjects of control have other types of characteristics such as for example an integral characteristic or oscillatory characteristic, adjustment of the control parameters in order to achieve a good control performance is difficult and, furthermore, in the case where the subjects of control have an unstable characteristic, adjustment of the control parameters becomes even more difficult.
Also, power-generating plants and many typical industrial processes (general industrial process) have the characteristic that, when the input of the subject of control changes, there is a certain time delay (dead time) before the output change takes place. There is therefore the problem that, even if the characteristic of the subject of control is taken to be a delay characteristic, if the dead time is long in comparison with the delay time constant, it is not possible to achieve sufficient control performance with the conventional PID control device.
Recently, therefore, in order to solve such problems, instead of a PID control device, a process control device has been proposed incorporating therein a positive feedback compensator comprising a first-order delay and dead time element. Such a device is disclosed in Laid-open Japanese Patent Publication No. P2002-157002, for example.
FIG. 1 is a block diagram showing an example of the construction of a prior art process control device comprising a positive feedback compensator and an external compensator of this type.
As shown in this Figure, the process control device comprises: a controller 11 that outputs in respect of a control subject (control target) 13 a process value (a control quantity) PV obtained by inputting the deviation e obtained by subtracting, using a subtractor 18, a setpoint value (a control target value) SV and a process value (a control quantity) PV of the control subject 13; an external compensator 12 that inputs the process value (the control quantity) PV of the control subject 13 and outputs a manipulative value in respect of the control subject 13; and a subtractor 14 that generates a final manipulative value (a final target control quantity) MV in respect of the control subject 13 by subtracting the manipulative value that is output by the controller 11 and the manipulative value that is output by the external compensator 12.
The controller 11 also comprises a compensator (positive feedback compensator) 15 comprising first-order delay and dead time, proportionator 16 and adder 17.
The proportionator 16 inputs the deviation e of the setpoint value SV and the process value PV of the control subject 13 and performs a proportional calculation; the adder 17 adds the output of this proportionator 16 and the output of the positive feedback compensator 15 and the manipulative value which is the output of the controller 11 is thereby obtained. The manipulative value which is the output of this controller 11 is fed back to this positive feedback compensator 15 as input.
As described above, the prior art process control device comprises an external compensator 12 in addition to the positive feedback compensator 15 so that not merely control subjects 13 having a long dead time but also control subjects 13 having an oscillatory characteristic or unstable characteristic, that cannot be satisfactorily controlled by PID control, may be satisfactorily controlled.
A process control device having such a positive feedback compensator 15 approximates the control subject 13 as first-order delay and a dead time element and determines the control parameters of the positive feedback compensator 15 by using these parameters.
Consequently, when an external compensator 12 is introduced, the characteristic of the entire closed loop system constituted by the control subject 13 and the external compensator 12 must be approximated as first-order delay and a dead time element.
As a result, every time the parameters of the external compensator 12 are altered, the approximate characteristic of the entire closed loop system constituted by the control subject 13 and the external compensator 12 must be calculated and the parameters of the positive feedback compensator 15 and proportionator 16 altered accordingly.
In order to obtain fully satisfactory control performance, an operation of adjusting this external compensator 12 on site is indispensable and it is necessary to repeat the three procedures:                (a) alteration of the parameters of the external compensator 12;        (b) characteristic approximation calculation of the closed loop system; and        (c) alteration of the parameters of the positive feedback compensator 15 and proportionator 16 until the prescribed control performance is obtained.        
However, in the case of power-generating plants or typical industrial processes, the subjects of control that must be adjusted may number as many as a few hundred and there is the problem that performing adjustment by repeating the three procedures described above in respect of all of these not only requires a long adjustment time but also may result in mis-setting of the parameters, due to increase in the number of times that the parameters must be altered.
In response to these problems, as shown in FIG. 2, a process control device may be provided comprising in the controller 11 an internal compensator 112 corresponding to the external compensator 12 of the process control device. This internal compensator 112 has a positive feedback compensation function and has the same construction as the external compensator 12 that performs proportional calculation and differential calculation, and the same parameters. The process control device may also comprise a model compensator 113 constituted by a model of the control subject 13, and, in addition, the parameters of the external compensator 12 and internal compensator 112 may be made the same.
Specifically, the construction of the process control device is made such that if, of the parameters of the external compensator 12 and internal compensator 112, either set of parameters is altered, the other set of parameters may also be automatically altered correspondingly, so that parameter alteration may be performed by a single adjustment operation. An example of such a process control device has been disclosed by the inventors of the present application as “Model-Driven PID Control System in Single-Loop Controller”.
Control elements having the same function as those described with reference to FIG. 1 are given the same reference symbols and further description thereof is dispensed with.
According to the present invention, it is unnecessary to approximate the characteristic of the closed loop system comprising the control subject 13 and external compensator 12 by a first-order delay and dead time element as illustrated in Laid-open Japanese Patent Publication No. P 2002-157002, so a single parameter alteration operation is sufficient. Consequently, the control subject may be satisfactorily controlled over a wider range in same way as the prior art process control device having a positive feedback compensator 15 shown in FIG. 1 described above and the operation of adjusting the control parameters of for example the external compensator 12, internal compensator 112 and model compensator 113 on site may be performed in a simple fashion.
However, in the latter proposal shown in FIG. 2 also, since the characteristic of the closed loop constituted by the control subject 13 and external compensator 12 is made to be a characteristic having a first-order delay and a dead time, setting of the compensation parameter of the external compensator 12 is performed by calculation, so there is a possibility of miscalculation occurring.
Also, in the case of a process having a large number of control subjects, there is a problem that not only is considerable time required to repeat the calculations individually until the prescribed control performance is obtained and, furthermore, to evaluate such control performance, but also that mistakes may occur in such human operations.
As described above, in power-generating plants and typical industrial processes, the number of control subjects that need to be controlled is extremely large and the characteristics of these control subjects are multifarious. A process control device is therefore demanded whereby the prescribed control performance in respect of control subjects having multifarious characteristics may be satisfactorily achieved.
Also, in order to perform adjustment in a short time and in a safe fashion, it is necessary that it should be possible to perform adjustment of the control parameters reliably by a simple procedure.
The present invention was made in view of the above problems. In a process control device having a positive feedback compensator capable of performing satisfactory control even in cases where the dead time is long compared with the time constant of the control subject or in cases where the characteristic of the control subject is not a lagging characteristic, its object is to provide a method of adjusting a process control device and adjustment tool therefor whereby the operation of on site adjustment of control parameters for obtaining satisfactory control performance may be performed in a short time and the appropriateness of the adjustment and control performance of the control parameters may be evaluated visually and whereby human miscalculation or misadjustment may be prevented.